Tumour necrosis factor (TNF)-stimulated gene 6 (TSG-6) is an inflammation-induced protein with protective roles in arthritis. TSG-6, the ˜35 kDa secreted product of TNF-stimulated gene-6, is expressed in response to inflammatory mediators and growth factors, where there is believed to be little or no constitutive expression of the protein in healthy tissues (Milner & Day (2003) J. Cell Sci. 116, 1863-1873).
There is increasing evidence that TSG-6, whilst induced in response to inflammation, has anti-inflammatory and chondroprotective properties, making it an endogenous inhibitor of joint destruction. In this regard, TSG-6 has been found to have diverse biological activities, such as inhibition of neutrophil migration, down-regulation of plasmin activity, and the cross-linking of hyaluronan (HA) chains, which are all likely to contribute to its chondroprotective properties (Wisniewski et al. (1996) J Immunol. 156, 1609-1615; and Milner et al, (2006) Biochem. Soc. Trans. 34, 446-450).
TSG-6, which is comprised almost entirely of contiguous Link and CUB_C domains, binds to a variety of protein and glycosaminoglycan ligands (including HA, chondroitin-4-sulphate, aggrecan, inter-α-inhibitor (IαI), pentraxin-3, thrombospondin-1, fibronectin and heparin/heparan sulphate), where the majority of these interactions are mediated through its Link module domain. Mutagenesis studies have revealed that at least three non-overlapping ligand-binding surfaces are present on the Link module (Mahoney et al. (2005) J. Biol. Chem. 280, 27044-27055; and Kuznetsova et al. (2005) J. Biol. Chem. 280, 30899-30908). To date, the only ligand identified for the CUB_C domain is fibronectin (D J Mahoney & A J Day, unpublished data). In addition, this domain contains a divalent cation-binding site (Rugg et al. (2005) J. Biol. Chem. 280, 25674-25686).
TSG-6 has been detected in the context of inflammatory diseases such as rheumatoid arthritis (RA), where it is present in the synovial fluid, cartilage and synovia. It is likely that TSG-6 is produced locally in joint tissues, since its expression can be induced in cultured human chondrocytes by TNF, IL-1, IL-6, TGF-β and PDGF and it is constitutively expressed by synoviocytes from RA patients, where its production is further enhanced by treatment with IL-1, TNF and IL-17 (Milner et al, (2006) Biochem. Soc. Trans. 34, 446-450).
Importantly, a number of recent studies have revealed that TSG-6 has a protective role in experimental models of arthritis. For example, in a model of collagen-induced arthritis (CIA; an autoimmune polyarthritis with a histopathology similar to human RA), there was delayed onset of symptoms and reduction of both disease incidence and joint inflammation/destruction in TSG-6 transgenic mice or wild-type mice treated systemically with recombinant human TSG-6 (Mindrescu et al. (2000) Arthritis Rheum. 43, 2668-2677; and Mindrescu et al. (2002) Arthritis Rheum. 46, 2453-2464). In TSG-6 transgenic animals, an ameliorative effect comparable to anti-TNF-antibody treatment was seen. Furthermore, in cartilage-specific TSG-6 transgenic mice, the instigation of antigen-induced arthritis (AIA; a model of monoarticular arthritis) resulted in delayed cartilage damage compared to controls, with reduced degradation of aggrecan by MMPs and aggrecanase, and there was evidence of cartilage regeneration, 4-5 weeks after the onset of disease in these animals (Giant et al. (2002) Arthritis Rheum. 46, 2207-2218). Similar chondroprotective effects were seen in wild-type mice where recombinant murine TSG-6 was injected directly into the affected joint in AIA or intravenously in proteoglycan-induced arthritis (PGIA; a model of human RA) (Bardos et al. (2001) Am. J. Pathol. 159, 1711-1721).
The anti-inflammatory and chondroprotective effects of TSG-6 observed in these studies are likely due to more than one mechanism. Most importantly, TSG-6 is a potent inhibitor of neutrophil extravasation in vivo and has also been implicated in the inhibition of the protease network through its potentiation of the anti-plasmin activity of IαI, where plasmin is a key regulator of proteolysis during inflammation, e.g., via its activation of MMPs (Wisniewski et al. (1996) J. Immunol. 156, 1609-1615; and Getting et al. (2002) J. Biol. Chem. 277, 51068-51076). In this regard, mice lacking TSG-6 develop an accelerated and much more severe form of PGIA than controls, with rapid and extensive cartilage degradation and bone erosion (Szántá et al. (2004) Arthritis Rheum. 50, 3012-3022). Increased neutrophil infiltration and plasmin activity were suggested to account for these effects in the TSG-6−/− mice.